Conventional refrigeration technology typically utilizes a heat pump that relies on compression and expansion of a fluid refrigerant to receive and reject heat in a cyclic manner so as to effect a desired temperature change or i.e. transfer heat energy from one location to another. This cycle can be used to provide e.g., for the receiving of heat from a refrigeration compartment and the rejecting of such heat to the environment or a location that is external to the compartment. Other applications include air conditioning of residential or commercial structures. A variety of different fluid refrigerants have been developed that can be used with the heat pump in such systems.
While improvements have been made to such heat pump systems that rely on the compression of fluid refrigerant, at best such can still only operate at about 45 percent or less of the maximum theoretical Carnot cycle efficiency. Also, some fluid refrigerants have been discontinued due to environmental concerns. The range of ambient temperatures over which certain refrigerant—based systems can operate may be impractical for certain locations. Other challenges with heat pumps that use a fluid refrigerant exist as well.
Magneto caloric materials (MCMs)—i.e. materials that exhibit the magneto caloric effect—provide a potential alternative to fluid refrigerants for heat pump applications. In general, and as used herein, the magnetic moments of a “normal” MCM will become more ordered under an increasing, externally applied magnetic field and cause the MCM to generate heat. Conversely, decreasing the externally applied magnetic field will allow the magnetic moments of the normal MCM to become more disordered and allow the normal MCM to absorb heat. Some MCMs exhibit the opposite behavior—i.e. generating heat when the magnetic field is removed—and such MCMs are referred to herein as para-magneto caloric or inverse MCMs. The theoretical Carnot cycle efficiency of a refrigeration cycle based on an MCM can be significantly higher than for a comparable refrigeration cycle based on a fluid refrigerant. As such, a heat pump system that can effectively use an MCM would be useful.
Challenges exist to the practical and cost competitive use of an MCM, however. In addition to the development of suitable MCMs, equipment that can attractively utilize an MCM is still needed. Provision should be made for the transfer or heat to and from the MCM, preferably in a continuous manner so that the equipment does not operate in a start and stop fashion that can be inefficient. Currently proposed equipment may require relatively large and expensive magnets, may be impractical for use in e.g., appliance refrigeration, and may not otherwise operate with enough efficiency to justify capital cost.
Additionally, as stated above, the ambient conditions under which a heat pump may be needed can vary substantially. For example, for a refrigerator appliance placed in a garage or located in a non-air conditioned space, ambient temperatures can range from below freezing to over 90° F. Some MCMs are capable of accepting and generating heat only within a much narrower temperature range than presented by such ambient conditions.
Accordingly, a heat pump or heat pump system that can address certain challenges such as those identified above would be useful. More particularly, such a heat pump that can effectively use MCMs would be particularly beneficial. Such a heat pump system that can also be used in e.g., a refrigerator appliance would also be useful.